Flexible, dicationic imidazolium salts for in situ application in palladium-catalysed Mizoroki–Heck coupling of acrylates under aerobic conditions

Article abstract of DOI:10.1002/aoc.3503

The synthesis, characterization and in situ catalytic performance of new unsymmetric N,N′-disubstituted imidazolium-based dicationic salts in Mizoroki–Heck coupling of acrylates with aryl bromides under aerobic conditions are described. A series of flexible dicationic salts with varying steric and electronic properties were synthesized in good to excellent yields. All the salts were well characterized using spectroscopic techniques. X-ray diffraction analysis of two salts with the same dicationic backbone and different counter anions shows that the ligand adopts two different conformations which are influenced by the nature of the anion. Thus, the ligand is capable of changing its conformation according to the change in environment due to its flexible nature. All the synthesized imidazolium salts were found to be active in in situ palladium-catalysed Mizoroki–Heck coupling under aerobic conditions. Amongst the salts, the hydroxyl-functionalized imidazolium salt, incorporating the features of both bidentate chelating O,O ligand and carbene, shows the maximum catalytic activity. A variety of aryl and heteroaryl methyl and ethyl cinnamates were synthesized using these imidazolium salts as preligands. In addition, NMR studies confirm in situ generation of normal N-heterocyclic carbenes from the C-2 position of imidazol-2-ylidene ring. The mercury poisoning test was also performed to ascertain the nature of catalytically active palladium species. Aerobic conditions, low catalytic loading (0.5?mol%), shorter reaction times, broad functional group tolerance and good to excellent isolated yields are some of the significant features of the novel catalytic systems described here. Copyright

Full text of DOI:10.1002/aoc.3503

Full paper
Received: 30 December 2015
Revised: 12 March 2016
Accepted: 10 April 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3503
Flexible, dicationic imidazolium salts for in situ
application in palladium-catalysed Mizoroki–Heck
coupling of acrylates under aerobic conditions†
Marilyn Daisy Milton* and Parul Garg
The synthesis, characterization and in situ catalytic performance of new unsymmetric N,N′-disubstituted imidazolium-based dica-
tionic salts in Mizoroki–Heck coupling of acrylates with aryl bromides under aerobic conditions are described. A series of flexible
dicationic salts with varying steric and electronic properties were synthesized in good to excellent yields. All the salts were well
characterized using spectroscopic techniques. X-ray diffraction analysis of two salts with the same dicationic backbone and differ-
ent counter anions shows that the ligand adopts two different conformations which are influenced by the nature of the anion.
Thus, the ligand is capable of changing its conformation according to the change in environment due to its flexible nature. All
the synthesized imidazolium salts were found to be active in in situ palladium-catalysed Mizoroki–Heck coupling under aerobic
conditions. Amongst the salts, the hydroxyl-functionalized imidazolium salt, incorporating the features of both bidentate chelat-
ing O,O ligand and carbene, shows the maximum catalytic activity. A variety of aryl and heteroaryl methyl and ethyl cinnamates
were synthesized using these imidazolium salts as preligands. In addition, NMR studies confirm in situ generation of normal
N-heterocyclic carbenes from the C-2 position of imidazol-2-ylidene ring. The mercury poisoning test was also performed to ascer-
tain the nature of catalytically active palladium species. Aerobic conditions, low catalytic loading (0.5 mol%), shorter reaction
times, broad functional group tolerance and good to excellent isolated yields are some of the significant features of the novel cat-
alytic systems described here. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: imidazolium salts; N-heterocyclic carbenes (NHCs); C–C bond forming reactions; Mizoroki–Heck coupling; palladium catalysis
Our preligand design strategy involved the synthesis of
unsymmetric N,N′-disubstituted imidazolium salts by introduction
of polyfunctionality which could offer (O,O)- or (N,N)-bidentate che-
Introduction
Palladium-catalysed Mizoroki–Heck coupling of olefins[1,2] is popu-
lar amongst organic chemists for carbon–carbon bond formation
due to its wide applications in the synthesis of useful pharmaceuti-
cals, conducting polymers, UV screens, natural products, etc.[3–5]
Traditional phosphine ligands[6,7] have limited synthetic utility
due to high cost, toxicity and sensitivity towards air and moisture.
However, in the last decade, a number of azolium salts functional-
ized with imidazoles, benzimidazoles and pyrazoles, N-heterocyclic
carbenes (NHCs), oxazolines, Schiff bases, pyridines and amines
have been developed as successful phosphine-free ligands for
Mizoroki–Heck reactions, including those incorporating azolium
salts into chelate as well as pincer scaffolds.[8–21] These ligands cir-
cumvent many limitations associated with the use of phosphine
igands, and also avoid excessive use of ligands and high loadings
of palladium. However, most of these require inert conditions or pre-
formation of palladium complexes for an effective Mizoroki–Heck
reaction. Therefore, development of novel ligands for direct in
situ application in palladium catalysis under aerobic conditions
is important.
lation. Dissymmetrization not only fine-tunes the steric and elec-
tronic properties of the ligand but also permits introduction of
chelatization, hemilability, polyfunctionality, shielding effects,
etc., which further influences catalyst stability, reactivity and
selectivity.[24] Furthermore, a pyridine ring, which is well known to
show cooperative effects[12,25–30] during catalysis, was introduced
on the ligand. However, the direct attachment of pyridine to imid-
azole unit would make the ligand structure rigid.[31] Therefore, the
pyridine ring was appended onto the imidazole nucleus through
a short methylene spacer which could release the strain and
increase the flexibility of the basic scaffold. Functional groups such
as hydroxyl and ammonium were introduced as the N-alkyl side
arms of the imidazole–pyridine scaffold with varied chain lengths
of the linkers to provide flexibility in chelation to the metal centre
during catalysis and increase the solubility in polar solvents.
Herein we describe the design and synthesis of a series of novel
unsymmetric N,N′-disubstituted imidazolium salts with varying
Also, it has been reported that charged ligands lead to increased
ionophilicity in comparison to the conventional neutral ligands and
can prevent the commonly encountered problems of leaching of
metal catalysts and difficulty in separation of metal catalyst from
reaction product.[21–23] Thus, we decided to design ligands incorpo-
rating dicationic imidazolium species for direct in situ application in
palladium-catalysed Mizoroki-Heck reaction.
*
Correspondence to: Marilyn Daisy Milton, Department of Chemistry, University of
Delhi, Delhi 110 007, India. E-mail: mdmilton@chemistry.du.ac.in
†
This paper is dedicated to Professor Sakae Uemura on the occasion of his 75th
birthday
Department of Chemistry, University of Delhi, Delhi 110 007, India
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

M. D. Milton and P. Garg
steric and electronic properties and their in situ catalytic perfor-
mance in the synthesis of methyl and ethyl cinnamates under
aerobic conditions.
Table 1. Crystallographic data for imidazolium salts 2 and 3
2
3
Empirical formula
Formula weight
Temperature (K)
Crystal system
C
13H19Br2N3O2
C13H19BrF6N3O2P
474.19
409.13
Experimental
298(2)
150(2)
Orthorhombic
Pna21
Monoclinic
P21/n
General
Space group
Unless otherwise noted, all reactions were carried out under aerobic
conditions. Solvents were dried according to standard procedures.
All chemicals were obtained from commercial sources and were
used without further purification. Reactions were monitored with
Merck Silica 60 F254 (0.040–0.063 mm) TLC plates. The 1H NMR
and 13C NMR spectra were recorded with a JEOL-JNM-ECX-400P
spectrometer using DMSO-d6. Infrared (IR) spectra were recorded
with a PerkinElmer FTIR Spectrum 2000 spectrophotometer using
KBr pellets. Melting points were determined with a Buchi M-560
melting point apparatus and are uncorrected. High-resolution
mass spectra were measured using a high-resolution TOF instru-
ment with electrospray ionization (ESI). The starting precursor
3-(1H-imidazol-1-ylmethyl)pyridine (1) was synthesized according
to an earlier reported procedure.[32]
Unit cell dimensions
a = 12.7725(11) Å
b = 5.4177(5) Å
c = 23.493(2) Å
α = 90°
a = 5.8667(3) Å
b = 28.275(2) Å
c = 10.9211(7) Å
α = 90°
β = 89.713(5)°
γ = 90°
β = 90°
γ = 90°
1625.6(3)
4
Volume (Å3)
1811.6(2)
4
Z
ρ
(calc) (mg mÀ3
)
1.672
1.739
Reflections collected
Independent reflections
R(int.)
4800
6891
2688
3109
0.0200
0.0147
Data/restraints/parameters
Final R indices [I > 2σ(I)]
2688/3/188
R1 = 0.0288
wR2 = 0.0603
R1 = 0.0354
wR2 = 0.0619
0.975
3109/2/241
R1 = 0.0302
wR2 = 0.0721
R1 = 0.0345
wR2 = 0.0739
1.048
R indices (all data)
X-ray Structure Analysis
GOF on F2
Single-crystal X-ray data were collected with an Oxford X Calibur
CCD diffractometer using graphite monochromated Mo Kα radia-
tion (λ = 0.71073 Å) at low temperature (150 K). The structures were
solved using SIR-92 and refined by the full matrix least squares tech-
nique on F2 using the SHELXL-97[33] program within the WinGX v.
1.70.01 software package. All hydrogen atoms were fixed at the
calculated positions with isotropic thermal parameters and all
non-hydrogen atoms were refined anisotropically. Crystal data
and details of the data collection are summarized in Table 1.
Largest difference peak
0.366
0.690
Hole (e ÅÀ3
)
À0.299
À0.378
stirred at room temperature for 4 days. Addition of methanol to
the same solution followed by stirring at room temperature for
24 h resulted in precipitation of a white solid. The solid was filtered,
washed with acetone and dried in vacuo to afford 3 as white solid.
Yield: 0.326 g (99%); m.p. 138–140 °C. IR (KBr, cmÀ1): 3336, 3060,
3036, 2973, 1569, 1561, 1507, 1477, 1450, 1414, 1157, 1065, 862,
Syntheses
1
835, 744, 558. H NMR (400 MHz, DMSO-d6, δ, ppm): 9.32 (s, 1H,
H2), 9.17 (s, 1H, H8), 9.01 (d, 1H, J = 6.6 Hz, H12), 8.62 (d, 1H,
J = 8.0 Hz, H10), 8.22-8.20 (m, 1H, H11), 7.82 (s, 1H, H5), 7.79 (s,
1H, H4), 5.69 (s, 2H, H6), 5.25 (t, 1H, J = 5.1 Hz, H15), 5.18 (t, 1H,
J = 5.1 Hz, H18), 4.64 (t, 2H, J = 5.1 Hz, H13), 4.21 (t, 2H, J = 5.1 Hz,
Synthesis of 3-(2-hydroxyethyl)-1-((1-(2-hydroxyethyl)pyridin-1-ium-3-yl)methyl)-
1H-imidazolium bromide (2)
A mixture of 1 (10.0 mmol, 1.596 g) and 2-bromoethanol
(30.4 mmol, 3.802 g) in dry acetonitrile (30 ml) was stirred at 50 °C
for 4 days. The precipitated solid was filtered and washed thrice
with acetonitrile. The solid was further purified by reprecipitation
from acetone–methanol affording 2 as a hygroscopic pale yellow
solid. Yield: 3.869 g (94%); m.p. 184–186 °C. IR (KBr, cmÀ1): 3305,
3272, 3135, 3066, 3033, 2927, 1637, 1574, 1560, 1501, 1451, 1406,
1379, 1174, 1065, 1054, 779, 687, 650. 1H NMR (400 MHz,
DMSO-d6, δ, ppm): 9.39 (s, 1H, H2), 9.26 (s, 1H, H8), 9.04 (d, 1H,
J = 6.6 Hz, H12), 8.67 (d, 1H, J = 8.0 Hz, H10), 8.25-8.21 (m, 1H,
H11), 7.89 (s, 1H, H5), 7.83 (s, 1H, H4), 5.75 (s, 2H, H6), 5.25–5.19
(br s, 2H, OH), 4.68 (t, 2H, J = 5.1 Hz, H13), 4.24 (t, 2H, J = 5.1 Hz,
H15), 3.88 (t, 2H, J = 5.1 Hz, H14), 3.73 (t, 2H, J = 5.1 Hz, H16).13C
NMR (100 MHz, DMSO-d6, δ, ppm): 145.48 (C10), 145.27 (C12),
145.16 (C8), 137.21 (C2), 134.87 (C9), 127.73 (C11), 123.38 (C4),
122.30 (C5), 63.36 (C13), 59.78 (C14), 59.08 (C16), 51.96 (C15),
48.03 (C6). HRMS (ESI-TOF) m/z: [MÀ2BrÀH]+; calcd for
C13H18N3O2: 248.1388, found: 248.1388.
H16), 3.85 (q, 2H, J = 5.1 Hz, H14), 3.71 (q, 2H, J = 5.1 Hz, H17). 13
C
NMR (100 MHz, DMSO-d6, δ, ppm): 145.45 (C10), 145.32 (C12),
145.10 (C8), 137.24 (C2), 134.96 (C9), 127.83 (C11), 123.44 (C4),
122.35 (C5), 63.52 (C13), 59.86 (C14), 59.14 (C17), 51.99 (C16),
48.20 (C6). HRMS (ESI-TOF) m/z: [M–BrÀPF6ÀC2H5O+H]+; calcd for
C11H15N3O: 205.1209, found: 205.0882.
Synthesis of 3-(3-hydroxypropyl)-1-((1-(3-hydroxypropyl)pyridin-1-ium-3-yl)methyl)-
1H-imidazolium bromide (4)
A mixture of 1 (4.42 mmol, 0.702 g) and 3-bromopropanol
(13.2 mmol, 1.835 g) in dry acetonitrile (20 ml) was stirred at 50 °C
for 4 days. The solvent was then removed in vacuo and the residue
was washed three times with acetonitrile. The resulting viscous res-
idue was stirred in hexane–acetone (2:1) at room temperature for
1 day. The solvent was then decanted off affording 4 as a highly
hygroscopic pale yellow solid. Yield: 1.390
g (72%); m.p.
132–134 °C. IR (KBr, cmÀ1): 3393, 3086, 2954, 2885, 1638, 1561,
Synthesis of 3-(2-hydroxyethyl)-1-((1-(2-hydroxyethyl)pyridin-1-ium-3-yl)methyl)-
1H-imidazolium bromide hexafluorophosphate (3)
1
1508, 1481, 1458, 1245, 1163, 1070, 949, 826. H NMR (400 MHz,
DMSO-d6, δ, ppm): 9.36 (s, 1H, H2), 9.26 (s, 1H, H8), 9.11 (d, 1H,
J = 6.6 Hz, H12), 8.61 (d, 1H, J = 8.0 Hz, H10), 8.21-8.19 (m, 1H,
H11), 7.86–7.84 (m, 2H, H5 and H4), 5.68 (s, 2H, H6), 4.68 (t, 2H,
A mixture of imidazolium salt 2 (0.69 mmol, 0.284 g) and potassium
hexafluorophosphate (1.38 mmol, 0.254 g) in water (10 ml) was
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Appl. Organometal. Chem. (2016)

Dicationic imidazolium salts for in situ Mizoroki-Heck coupling in air
J = 7.3 Hz, H13), 4.25 (t, 2H, J = 7.3 Hz, H17), 3.48–3.42 (m, 4H, H15
and H19), 2.12–2.06 (m, 2H, H14), 1.99–1.92 (m, 2H, H18). 13C NMR
(100 MHz, DMSO-d6, δ, ppm): 145.83 (C10), 145.50 (C12), 145.26
(C8), 137.42 (C2), 135.56 (C9), 128.83 (C11), 123.83 (C4), 123.12
(C5), 59.86 (C13), 57.90 (C15), 57.86 (C19), 49.04 (C6), 47.47 (C17),
33.47 (C14), 32.41 (C18). HRMS (ESI-TOF) m/z: [MÀ2BrÀH]+; calcd
for C15H22N3O2: 276.1707; found: 276.1762.
The characterization data for compounds 9 and 10 were in entire
agreement with those reported in the literature.[8–21,34]
General Procedure for NMR Tube Reactions
Compounds 7a (0.062 mmol) and 8a (0.0125 mmol) were
added to a mixture of imidazolium salt 2 (0.062 mmol), Et3N
(0.062 mmol) and PdCl2 (0.062 mmol) in DMSO-d6 (0.5 ml).
The reaction mixture was stirred at 130 °C and monitored
using 1H NMR and 13C NMR spectroscopy at different time
intervals.
Synthesis of 3-(3-ammoniopropyl)-1-((1-(3-ammoniopropyl)pyridin-1-ium-3-yl)
methyl)-1H-imidazolium bromide (5)
A mixture of 1 (1.13 mmol, 0.180 g) and 3-bromopropylamine
hydrobromide (1.03 mmol, 0.226 g) in dry acetonitrile (5 ml) was
stirred at room temperature for 2 days. The solvent was then
decanted off and the viscous residue was thoroughly washed with
acetonitrile. The residue was further purified by reprecipitation
from chloroform–methanol affording 5 as a dark brown viscous liq-
uid. Yield: 0.307 g (97%). IR (KBr, cmÀ1): 3413, 3043, 2046, 1623,
1478, 1167, 751. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 9.58 (s, 1H,
H2), 9.50 (s, 1H, H8), 9.22 (d,1H, J = 5.9 Hz, H12), 8.75 (d, 1H,
J = 8.1 Hz, H10), 8.28-8.25 (m, 1H, H11), 8.02–7.93 (m, 8H, H5, H4,
H16 and H20), 5.78 (s, 2H, H6), 4.80 (t, 2H, J = 6.6 Hz, H13), 4.35 (t,
2H, J = 6.6 Hz, H17), 2.89 (br s, 4H, H15 and H19), 2.32–2.29 (m,
2H, H14), 2.18–2.14 (m, 2H, H18). 13C NMR (100 MHz, DMSO-d6, δ,
ppm): 146.22 (C10), 145.24 (C12 and C8), 137.57 (C2), 135.59 (C9),
128.96 (C11), 123.47 (C4), 123.28 (C5), 58.69 (C13), 48.99 (C6),
46.95 (C17), 36.21 (C19), 36.04 (C15), 28.72 (C14), 27.6 (C18). HRMS
(ESI-TOF) m/z: [MÀ3BrÀH]+; calcd for C16H30BrN5: 371.1685; found:
371.1009.
General Procedure for Mercury Poisoning Test
In a typical experiment, aryl halide (5.0 mmol) and alkyl acrylate
(10.0 mmol) were added to a mixture of imidazolium salt 2
(0.5 mol%), Et3N (5.0 mmol) and PdCl2 (0.5 mol%) in DMF
(2 ml). The reaction mixture was stirred at 130 °C for 5 min. After
5 min, 300 equiv. of Hg(0) was added and the reaction mixture
was stirred at 130 °C for 4 h. On completion, the reaction mixture
was treated with water and extracted with ethyl acetate
(3 × 10 ml). The combined organic extracts were dried over an-
hydrous Na2SO4, filtered and evaporated. The residue was then
purified by column chromatography on silica gel using petro-
leum ether–ethyl acetate as the eluent to give the desired
product.
Results and Discussion
Synthesis of 3-ethyl-1-((1-(ethyl)pyridin-1-ium-3-yl)methyl)-1H-imidazolium bro-
mide (6)
The synthetic methodology employed for the unsymmetric
imidazolium salts is outlined in Scheme 1. The reaction of 1 with
2-bromoethanol and 3-bromopropanol in dry acetonitrile afforded
the hydroxyl-functionalized imidazolium salt 2 and its homologue
4, respectively, as pale yellow hygroscopic solids in good to excel-
lent yields (72–94%). The ammonium-functionalized salt 5 was
isolated as a dark brown viscous liquid after reaction of 1 with
3-bromopropylamine hydrobromide at room temperature in excel-
lent yield (97%). Further, bromide counter-anion exchange in 2 with
2 equiv. of hexafluorophosphate afforded 3 as a slightly hygro-
scopic white solid in 99% yield. The bromide counter-anion
exchange was confirmed using 1H NMR and 13C NMR spectroscopy.
A mixture of 1 (4.03 mmol, 0.642 g) and ethyl bromide (12.18 mmol,
1.328 g) in dry acetonitrile (20 ml) was stirred at 50 °C for 4 days. The
precipitated solid was filtered and washed thrice with acetonitrile.
The solid was purified by reprecipitation from acetone–methanol
affording 6 as a light brown hygroscopic solid. Yield: 0.972 g
(64%); m.p. 228–230 °C. IR (KBr, cmÀ1): 3055, 3021, 2979, 1574,
1500, 1475, 1461, 1173, 872, 797, 752, 686. 1H NMR (400 MHz,
DMSO-d6, δ, ppm): 9.44 (s, 1H, H2), 9.35 (s, 1H, H8), 9.13 (d, 1H,
J = 6.4 Hz, H12), 8.63 (d, 1H, J = 8.7 Hz, H10), 8.21-8.17 (m, 1H,
H11), 7.89–7.82 (m, 2H, H5 and H4), 5.68 (s, 2H, H6), 4.63 (q, 2H,
J = 7.3 Hz, H13), 4.18 (q, 2H, J = 7.3 Hz, H15), 1.53 (t, 3H,
J = 7.3 Hz, H14), 1.41 (t, 3H, J = 7.3 Hz, H16). 13C NMR (100 MHz,
DMSO-d6, δ, ppm): 145.81 (C10), 145.33 (C12), 145.07 (C8), 137.25
(C2), 135.69 (C9), 128.60 (C11), 123.18 (C4), 123.08 (C5), 57.13
(C13), 48.69 (C6), 45.01 (C15), 16.71 (C14), 15.38 (C16). HRMS
(ESI-TOF) m/z: [MÀ2BrÀH]+; calcd for C13H18N3: 216.1490; found:
216.1493.
General Procedure for Mizoroki–Heck Reaction
In a typical experiment, aryl halide (5.0 mmol) and alkyl acrylate
(10.0 mmol) were added to a mixture of imidazolium salt 2
(0.5 mol%), Et3N (5.0 mmol) and PdCl2 (0.5 mol%) in
dimethylformamide (DMF; 2 ml). The reaction mixture was stirred
at 130 °C for 4 h. On completion, the reaction mixture was treated
with water and extracted with ethyl acetate (3 × 10 ml). The com-
bined organic extracts were dried over anhydrous Na2SO4, filtered
and evaporated. The residue was then purified by column chroma-
tography on silica gel using petroleum ether–ethyl acetate as the
eluent to give the desired product.
Scheme 1. Synthesis of imidazolium salts 2–6. Reagents and conditions: (a)
BrCH2CH2OH, CH3CN, 50°C; (b) KPF6, H2O–MeOH, r. t.; (c) BrCH2(CH2)2X,
CH3CN, 50 °C or r.t.; (d) BrCH2CH3, CH3CN, 50°C.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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M. D. Milton and P. Garg
The characteristic singlet corresponding to NCHN proton of
imidazolium ring showed an upfield shift from 9.39 ppm in 2 to
9.31 ppm in 3. The presence of PF6À anion in 3 was further con-
firmed by the appearance of a very strong absorption peak at
835 cmÀ1 in the IR spectrum. However, the fact that only one
counter-anion exchange had occurred was confirmed using
single-crystal X-ray studies. The imidazolium salt 6, which is a
non-functionalized analogue of 2, was synthesized similarly in good
yield (64%).
packed in a symmetric fashion such that the imidazolium rings of
one layer oriented themselves in face-to-face orientation with those
of the parallel layer at a separation of 5.418–5.867 Å with no π–π
stacking interactions (Figs S1–S4, supporting information). Thus,
the counter-anion exchange revealed the high flexibility of the
designed dicationic ligand backbone, in adopting different shapes
according to the environment, which can be exploited for coordina-
tion to a metal centre during catalysis.
Mizoroki–Heck Reaction
X-ray Diffraction Studies
We began our work with the coupling of 4-bromobenzonitrile (7a)
with methyl acrylate (8a) under a set of chosen reaction conditions.
Initially, the reaction was performed in the absence of any ligand,
which furnished only 65% yield of methyl (E)-3-(4-cyanophenyl)ac-
rylate (9a) (Table 2, entry 1). Addition of PPh3 enhanced the yield to
71% (Table 2, entry 2). The starting precursor 1 furnished 80% yield
of 9a (Table 2, entry 3). Next, the imidazolium salts (2–6) were
tested as preligands in the chosen model Mizoroki–Heck reaction
(Table 2, entries 4–8). Different reactivities were observed for the
structurally different imidazolium salts. Use of the hydroxyethyl-
functionalized imidazolium salt 2 in combination with PdCl2
resulted in the formation of a homogeneous orange-coloured solu-
tion and significantly increased the activity of palladium catalyst,
affording the coupling product 9a in 96% yield at low catalytic load-
ing of 0.5 mol% in a short reaction time of 4 h (Table 2, entry 4).
However, use of imidazolium salt 3, with similar imidazolium skele-
ton but heterogeneous counter anions, resulted in slight precipita-
tion of palladium black affording 92% yield of 9a (Table 2, entry 5).
Also, the length of the alkyl chain of N-substituents in imidazolium
salts had a direct influence on the reactivity and stability of the in
situ generated catalyst. The presence of propyl group in place of
ethyl group in imidazolium salt 4 resulted in very significant palla-
dium black precipitation, and hence reduced product yield (70%;
Table 2, entry 6). Also, the ammonium-functionalized imidazolium
Crystals of 2 and 3 suitable for X-ray diffraction analysis were grown
from methanol–ethyl acetate (1:1) mixture. Compound 2 crystal-
lized in orthorhombic space group Pna21 whereas 3 crystallized
in monoclinic space group P21/n. In 2, the ligand backbone
adopted a helical shape with the two hydroxyl protons of the same
molecule being involved in hydrogen bonding interactions with
bromide ions of different molecules (2.391 Å). In contrast, in 3 the
ligand backbone adopted a folded shape. Exchange of one bro-
mide counter anion with hexafluorophosphate flipped the orienta-
tion of the hydroxyl groups in 3 (Fig. 1). As a result, both the
hydroxyl protons moved closer to interact with the same bromide
ion in 3. The BrÀ anion showed HÁÁÁBr bonding interactions with
both hydroxyl protons H1 (2.421 Å) and H2 (2.423 Å) whereas PF6À
formed HÁÁÁF bonds with H9 (2.497 Å) of imidazolium ring and H5
(2.494–2.589 Å) of pyridinium ring. Additionally, the molecules
Table 2. Screening of imidazolium salts in model Mizoroki–Heck
reactiona
Entry
Ligand
Yield (%)b
—
1
2
3
4
5
6
7
8
65
71
80
96
92
70
83
90
PPh3
1
2
3
4
5
6
aReaction conditions: 7a (5.0 mmol), 8a (10.0 mmol), Et3N (5.0 mmol),
DMF (2.0 ml), 130°C, 4 h, under aerobic conditions.
bIsolated yield.
Figure 1. Molecular structures of imidazolium salts 2 and 3 before and
after counter-anion exchange, respectively (50% probability ellipsoids). The
dotted lines represent the hydrogen bonding interactions.
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Dicationic imidazolium salts for in situ Mizoroki-Heck coupling in air
salt 5 produced 9a in only 83% yield (Table 2, entry 7). Interestingly,
the imidazolium salt 6, which is a non-functionalized analogue of 2,
also underwent an effective coupling of 90% despite very great pre-
cipitation of palladium black (Table 2, entry 8). These observations
suggested that the availability of multiple ligations in 2 could be
a reason for the excellent performance of the in situ generated pal-
ladium catalyst. Also, the presence of hydroxyl functional group
and appropriate length of the alkyl chain of N-substituents in
imidazolium salts are crucial for their activity. Importantly, all the
ligands exhibited complete regioselectivity in producing the
trans-isomer exclusively with no cis- or gem-coupling product
formation. So, imidazolium salt 2 was used as preligand for
further studies.
(Table 3, entries 2–5). Strong bases such as NaOH and KOH afforded
low yields due to hydrolysis of the ester product (78 and 39%,
respectively; Table 3, entries 6 and 7). So, Et3N remained to be the
base of choice. Furthermore, a base-free reaction afforded only
5% yield giving strong evidence for the crucial role played by the
base in such coupling reactions (Table 3, entry 8).
Investigation of solvents showed DMF to be the most productive.
N-methyl-2-pyrrolidone (NMP) as solvent afforded 90% conversion
(Table 3, entry 9). The reactions in other solvents such as CH3CN,
H2O, MeOH and DMSO resulted in large amounts of palladium
black precipitation, and thus much reduced yields of 9a were ob-
served (2–33%; Table 3, entries 10–13). Any decrease or increase
in the volume of DMF reduced the yield (93 and 72%, respectively;
Table 3, entries 14 and 15). Further shortening of the reaction time
from 4 to 2 h reduced the yield to 91% (Table 3, entry 16). Also,
extending the reaction time to 8 h resulted in ca 10% decrease in
the isolated yield (87%; Table 3, entry 17). Other palladium salts
such as Pd(OAc)2, Pd2(dba)3 and PdCl2(CH3CN)2 were also exam-
ined; however, maximum catalytic performance was observed with
PdCl2 in combination with imidazolium salt 2 at 0.5 mol% catalytic
loading (Table 3, entries 18–20). An attempt to replace palladium
salt with comparatively cheaper Ni(OAc)2Á4H2O did not result in
any coupling reaction. Further reduction in catalytic loading to
0.1 mol% reduced the yield to 52% (Table 3, entry 21). So, the best
conversion was achieved in the reaction of 4-bromobenzonitrile
with methyl acrylate in the presence of Et3N as base using
0.5 mol% loading of PdCl2 and imidazolium salt 2 in 2 ml of DMF
at 130 °C in 4 h under aerobic conditions. It is noteworthy that
under these reaction conditions, 4-bromobenzonitrile could be
coupled in excellent yield without any poisoning of the catalyst in
contrast to phosphine-based preligands reported by Hong and
co-workers under inert atmosphere.[35]
The other reaction conditions such as base, solvent and time
were also subjected to optimization. A variety of organic and inor-
ganic bases were examined (Table 3). Use of weak bases such as
NaHCO3, Na2CO3, NaOAc and K2CO3 afforded 47–92% yields of 9a
Table 3. Optimization of Reaction Conditions using model Mizoroki-
Heck reactiona
Entry
Pd catalyst
PdCl2
Base
Solvent Temp. (°C) Yield (%)b
1
Et3N
DMF
130
130
130
130
130
130
130
130
130
80
96
47
92
86
81
78
39
5
2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
Pd(OAc)2
Pd2(dba)3
NaHCO3 DMF
3
Na2CO3
NaOAc
K2CO3
NaOH
KOH
—
DMF
DMF
DMF
DMF
DMF
DMF
NMP
CH3CN
H2O
Mizoroki–Heck coupling reactions of aryl bromides with acrylates
4
After optimizing the reaction conditions, various aryl bromides
were examined in the coupling reaction with methyl and ethyl
acrylate (Table 4). The coupling reaction proceeded smoothly with
aryl bromides bearing electron-withdrawing groups affording mod-
erate to excellent yields of the corresponding methyl and ethyl
cinnamates (Table 4, entries 1–12). A variety of functional groups
such as CN, NO2, CHO, COCH3, COPh and CF3 on the aryl bromides
were well tolerated. Different reactivities were observed with ortho-,
meta- or para-isomers, the ortho-derivatives giving generally lower
conversions (Table 4, entries 3–5 and 6–8). These results are in
agreement with general observations related to lower reactivity of
ortho-substituted aryl halides in such cross-coupling reactions for
steric reasons.[36] However, the reaction of o-bromobenzaldehyde
with acrylates afforded unexpected doubly substituted deformylated
products along with the expected o-formyl cinnamates.[34] The
heteroaryl bromides 2-bromothiophene and 3-bromopyridine
furnished low to moderate yields of the coupling products
(Table 4, entries 13 and 14). The aryl bromide without any addi-
tional functional group on benzene ring afforded the correspond-
ing methyl cinnamate in only16% yield (Table 4, entry 15).
Furthermore, the presence of electron-donating groups such as
OCH3 and CH3 in benzene ring deactivated the coupling with
acrylates resulting in no product formation. Even increased reac-
tion time (up to 24 h) or higher catalytic loading (5 mol%) could
not enhance the reactivity of deactivated aryl bromides. Also, no
product formation occurred with 4-chlorobenzonitrile, owing to
the high bond strength of the C–Cl bond.
5
6
7
8
9
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
90
2
10
11
12
13
14c
15d
16e
17f
18
19
20
21g
100
80
6
MeOH
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
11
33
93
72
91
87
91
87
87
52
130
130
130
130
130
130
130
130
130
PdCl2(CH3CN)2 Et3N
PdCl2 Et3N
aReaction conditions: 7a (5.0 mmol), 8a (10.0 mmol), Pd salt (0.5 mol %),
imidazolium salt 2 (0.5 mol%), base (5.0 mmol), solvent (2 ml), 4 h, under
aerobic conditions.
bIsolated yield.
c1 ml of DMF.
d4 ml of DMF.
e2 h.
f8 h.
g0.1 mol% catalytic loading.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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M. D. Milton and P. Garg
Table 4. Scope of aryl bromides in coupling with methyl and ethyl
Mizoroki–Heck reaction of 4-bromobenzonitrile with methyl acrylate.
acrylatea
1H NMR studies
In order to explore the interaction of the imidazolium salt 2 with the
palladium centre, the reaction of 7a with 8a was performed in
DMSO-d6 in an NMR tube in the presence of Et3N at 130 °C for 2 h
under aerobic conditions using stoichiometric amounts of PdCl2
and imidazolium salt 2. The reaction was monitored using 1H
NMR and 13C NMR spectroscopy (Fig. 3).
Entry
7
Yield (%)b
1
After 2 h, the H NMR spectrum displayed a new set of signals
R = Me
R = Et
parallel to those of the imidazolium salt 2 wherein the new signal
for the most acidic H2 proton of imidazol-2-ylidene ring was con-
spicuously absent from the newly generated set of signals, thus
pointing towards the most plausible coordination of the C2
carbenic carbon of imidazol-2-ylidene ring to palladium. Further,
1H NMR spectroscopy revealed that the NHC was bound in the nor-
mal mode: the 1H NMR spectrum of the newly generated catalytic
species showed two new singlets at 7.53 and 7.36 ppm, typical
for H4 and H5 present at the back side of the imidazol-2-ylidene
ring.[37] It is noteworthy that the NCHN proton signal did not disap-
pear in the absence of PdCl2 under the same reaction conditions at
130 °C in DMSO-d6. Moreover, the 13C NMR analysis of reaction mix-
ture showed a new signal at 165.9 ppm, which can be attributed to
s
4-NCPhBr
96
79
95
84
54
90
53
28
84
30
75
84
32
42
16
94
89
96
98
25
77
53
26
65
26
22
39
36
73
—
2
3-NCPhBr
3
4-O2NPhBr
3-O2NPhBr
2-O2NPhBr
4-OHCPhBr
3-OHCPhBr
2-OHCPhBr
4-H3COCPhBr
3-H3COCPhBr
4-PhOCPhBr
4-F3CPhBr
4
5
6
7
8c
9
10
11
12
13
14
15
2-Bromothiophene
3-Bromopyridine
PhBr
Ccarbene–Pd bond formation (supporting information).Thus, NMR
studies confirmed the in situ generation of normal NHCs from the
C-2 position of imidazol-2-ylidene ring and the formation of
aReaction conditions: 7 (5.0 mmol), 8a/8b (10.0 mmol), PdCl2 (0.5 mol%),
imidazolium salt 2 (0.5 mol%), Et3N (5.0 mmol), DMF (2 ml).
C
carbene–Pd bond.
bIsolated yield.
cNMR calculated yield.
Mercury Poisoning Test for Imidazolium Salt 2
To further investigate the nature of catalytically active palladium
species, we performed the mercury poisoning test by adding Hg
(0) in excess, which amalgamates with Pd(0) and suppresses its cat-
alytic activity.[38] Thus, in a representative experiment, the reaction
between 4-bromobenzonitrile and methyl acrylate was carried out
under standard conditions employing PdCl2–salt 2 in the presence
of a large excess of elemental mercury (in a molar ratio of 300:1
Hg/Pd). Hg(0) was added after 5 min of reaction and the reaction
mixture was stirred for 4 h. A yield of 84% was isolated in compar-
ison to 96% formed in an analogous experiment without Hg(0),
thus showing formation of stable homogeneous palladium cata-
lysts with imidazolium salt 2 which continued the reaction even
after the addition of mercury. However, the partial decrease in cat-
alytic activity in the presence of Hg(0) points towards the formation
of small amounts of Pd (0) nanoparticles in the reaction mixture.
Mizoroki–Heck reaction of 4-bromobenzonitrile with methyl acrylate.
Recycling of catalyst
The recovery and reuse of catalysts are highly desirable aspects of
metal-catalysed reactions due to the high cost of metal catalysts,
as well as for prevention of contamination of products with toxic
metals. The recycling ability of the PdCl2–imidazolium salt 2 cata-
lytic system was investigated in the Mizoroki–Heck reaction of
4-bromobenzonitrile with methyl acrylate in DMF at 130 °C for 4 h
(Fig. 2). After the first cycle, the product was extracted with ethyl
acetate (3 × 10 ml). The water layer comprising PdCl2/imidazolium
salt/Et3NH+BrÀ left after the first run was dried and charged with
fresh substrate and base. Again, the coupling reaction was per-
formed under the same experimental conditions.
The catalytic system showed a decrease in catalytic activity with
each run without any palladium black precipitation. Thus, the
reduced product yield could be due to increased viscosity of the
reaction mixture caused by accumulation of the salt formed from
Et3N after each run.
Figure 2. Recycling of catalyst in model Mizoroki–Heck reaction. Reagents
and conditions: PdCl2 (0.5 mol%), imidazolium salt 2 (0.5 mol%), Et3N, DMF,
130°C, 4 h.
Figure 3. Partial 1H NMR spectra recorded in DMSO-d6 at (a) 0 h and (b) 2 h.
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Copyright © 2016 John Wiley & Sons, Ltd. Appl. Organometal. Chem. (2016)

Dicationic imidazolium salts for in situ Mizoroki-Heck coupling in air
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These results are also in agreement with the NMR studies, which
also showed formation of in situ Pd–NHC complex during the
catalytic reaction.
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Conclusions
The synthesized imidazolium salts were found to be efficient
phosphine-free preligands for palladium-catalysed Mizoroki–Heck
reaction under aerobic conditions. The nature of the imidazolium
salts had a noticeable impact on their catalytic performance. The
hydroxyl-functionalized imidazolium salt 2, incorporating the fea-
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fully catalysed the Mizoroki–Heck reactions for a variety of aryl
bromides with high activity and stability under aerobic conditions.
Also, the catalytic system showed good tolerance to a variety of
functional groups present in aryl bromides but failed to catalyse
the reaction with aryl chlorides. The good to excellent yields iso-
lated for a variety of methyl and ethyl cinnamates using low cata-
lytic loadings under aerobic conditions in a short reaction time
provide evidence for the practical utility of these catalytic systems.
All the imidazolium salts used in this study produced the trans-
product selectively; therefore, their use can be extended to other
regio- or stereoselective reactions. The NMR investigations with
the hydroxyl-functionalized salt showed in situ generation of NHC
from C-2 position of imidazol-2-ylidene ring and the formation of
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Acknowledgements
We gratefully acknowledge financial support from the University
of Delhi, India. P.G. also acknowledges the Council for Scientific
and Industrial Research, India for Junior and Senior Research
Fellowships. The authors are grateful to USIC-CIF, University of
Delhi for single-crystal X-ray diffraction, HRMS and NMR spectral
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